Multi-turn angle measuring devices are electromechanical sensors which detect absolutely several revolutions; i.e., within the resolution of the device the distinct angular position is always available even after the device has been switched on and completed several revolutions. Such angle measuring devices are used to measure shafts and axles and for positioning and control of electric motors in industry, especially in connection with manufacturing and handling machinery.
Multi-turn shaft encoders preferably have a compact construction and a high number of distinctly detectable turns. DE 28 17 172 C2 describes a shaft encoder which has a compact footprint in that the multi-turn dimensional standards are driven by a spur reduction gear and lie within the circumferential region of the single code disk. In this case, the sensors for detection of the multi-turn and the single-turn rotational movements, or scales, are arranged on different circuit boards.
DE 34 29 648 C2 describes a multi-turn shaft encoder which is used to scan an optical single code disk and another optical multi-turn code disk which are arranged within the circumferential region of the single code disk, which are arranged on one circuit board. Here as well, spur gears with axes of rotation parallel to that of the input shaft are used for the reduction.
A multi-turn shaft encoder is also disclosed in DE 100 60 574 A1, which uses detectors arranged on a circuit board for the detection of single and multi-turn code carriers.
In the turn counter described in DE 100 01 676 C1, reed contacts arranged along a circumferential region of an input shaft are activated by means of a magnet rotating with the input shaft and generating counting pulses. Such so-called counting multi-turn shaft encoders, however, require a backup battery for counting when power is off. The reed contacts are subject to mechanical wear and therefore have only a limited lifetime. Moreover, the battery takes up space and must therefore be positioned outside the sensor housing when the size of the multi-turn shaft encoders must be kept small.
The reduction gearings of known mechanical multi-turn shaft encoders are configured as spur gearings with gear turning axes that are parallel to the input shaft. When the input shaft has a large diameter, therefore, the driven gear of the first multi-turn stage must have an even larger diameter than the one mounted on the input shaft in order to accomplish a reduction. However, this requires a much larger footprint. Otherwise the rotational speed of the first driven gear will be even higher than the rotational speed for the gear fixed to the input shaft. Accordingly, a high rotational speed of the input shaft results in greater wear of the gears which leads to a reduced lifetime for the multi-turn angle measuring device.